What is the principle of EPR measurement?
EPR polarimetry measures the frequency shift of Rb Zeeman resonance caused by polarized 3He inside pumping chamber. The absolute frequency of Rb Zeeman resonance is determined by both the holding field (25 Gauss) and a small field caused by the polarization of 3He (about 0.1 Gauss). Because the holding field fluctuates at a level of 10E-3, it is necessary to reverse the direction of 3He polarization (usually this is done by AFP). By measuring the frequency difference between these two polarization directions, the instability of holding field is cancelled so one is able to extract 3He polarization. • How do we measure Rb Zeeman resonance frequency? To detect the EPR resonance, we rely on the fact that during optical pumping the polarization of Rb vapor is very high. Most of the Rb atoms are in the F=3, M=-3 state (or M=+3 state for oppositely polarized light). Laser light can penetrate quite far into the cell because most Rb atoms are in the state which cannot absorb circularly polarize
